This invention relates generally to integrator circuits and, more particularly, to gated integrator circuits capable of high precision, ultrafast operation.
Although practical integrator circuits are simple in concept and straightforward in theoretical operation, various practical problems arise when actual circuits are built and operated. Frequently, the signal to be integrated includes a DC component or baseline offset that, if not subtracted or otherwise compensated for, distorts the final result. In addition, electronic switches do not simply open and close perfectly but, rather, inject spurious charges of their own that can adversely affect integrating accuracy. Active components, such as op amps, introduce inaccuracies of their own. Passive components, such as resistors and capacitors, are non-ideal and can introduce further inaccuracy.
Depending on the application, it is often possible to compensate for particular sources of inaccuracy and obtain reasonable performance over a limited range. However, accuracy is typically obtained at the expense of high speed operation and vice-versa. Generally, accurate circuits tend to be slow, and fast circuits tend to be inaccurate.
In the past, compensation networks were added to each electronic switch to reduce switch charge injection errors. Although effective, this approach was problematic in that it was extremely difficult to trim all the networks' components to obtain accurate compensation, particularly when large quantities of integrators were produced. Similarly, voltage-to-current converters (i.e., current pumps) were placed in front of the electronic gate switches to minimize the error introduced by the "on" resistance of the switches. These current pumps became unstable and temperature dependent, however, when they were used with high speed op amps. Signal baseline error has been corrected in the past by first integrating the total input signal during a first integrating period or window and then integrating the signal baseline offset over a second integrating period or window. The two integrated signals were then subtracted to yield the final result. Although effective in theory, practical circuits employing such an approach were very complicated. As a result, it was difficult to match the characteristics of the two gated integrators and compensate for other errors such as charge injection errors. Again, although some success has been achieved in improving integrator circuit operation under some conditions, accuracy has not heretofore been obtained in combination with extreme circuit speed and reasonable circuit simplicity and economy.